This invention relates to production of polyester textile yarn and is more particularly concerned with continuous filament, textured yarn.
Most polyester textile yarn is produced by processes which involve melt-spinning an ethylene terephthalate polymer to form continuous filaments, drawing the filaments to provide improved tenacity and a break elongation of about 30 percent, and treating the filaments to provide bulk with low boil-off shrinkage in yarns comprising the filaments. A boil-off shrinkage of less than 3.5 percent is important for uniform dyeing of yarn packages wound without special precautions. In one conventional process, the drawn filaments are stuffer-box crimped, heat-set to stabilize the crimp and reduce the boil-off shrinkage to about 8 percent, cut to staple fiber, blended with cotton or wool, and spun into yarn by a combination of carding, drafting and twisting steps. In another conventional process, the drawn continuous filaments are gathered into a yarn which has about 8 to 10 percent boil-off shrinkage, and the yarn is then textured with an air-jet in the manner disclosed in Breen U.S. Pat. No. 2,852,906. The filaments are formed into convolutions which impart bulk and provide crunodal loops on the yarn surface which impart to fabrics an aesthetic feel similar to that provided by yarns prepared by cutting to staple fibers and spinning the fibers into yarn. In package dyeing such air-jet textured yarns, special precautions are taken in winding to produce very soft packages which allow uniform dyeing. Alternatively, the yarn may be relaxed by hot stabilization treatment, but this is costly.
In another process for texturing yarn, the filaments are helically crimped by twisting the yarn, heatsetting the twist in the yarn at temperatures above 180.degree.C. and then untwisting the yarn. The yarn is advanced continuously over a heater to a false-twisting spindle or frictiontwisting device which backs up twist in the yarn passing over the heater. After the texturing step, the untwisted yarn is usually passed over a second heater at a slightly lower temperature than the first heater. The second heater reduces crimp contraction upon boil-off of the false-twist textured yarn, thus permitting the use of relatively firmly wound packages in package dyeing. Otherwise, excessively soft packages must be wound under low tension to permit the yarn to shrink uniformly. However, the second heating step also increases the cost of the texturing machine, maintenance and energy consumption.